Titration of a Weak Base Experiment

Titration of a Weak Base Experiment

Crystal Roberts

Mr. Thurston

SCH4U

November, 24th, 2015

Titration of a Weak Base

Introduction

A titration is when one solution of an acid is added to specific volume of another unknown base until a colour change occurs which indicates the pH change (van Kessel., et al., 2003). Kw, is the ionization constant for water, represented as 1 x 10-14 at SATP, and is the relationship between pH and pOH, which always stays constant (van Kessel., et al., 2003). Ka, is the acid ionization constant in an equilibrium constant for the ionization of an acid. Kb, is the base ionization constant in the equilibrium constant for ionization of a base in an aqueous solution (Kb and Ka).  A strong acid is an acid that ionizes completely, forming an aqueous solution. A strong base, on the other hand is an ionic substance that dissolves completely in water, releasing hydroxide ions (van Kessel., et al., 2003). A weak acid is one which does not ionize fully when it is dissolved in water, not being able to release all of its hydrogen ions (Clerk, J. 2013). A weak base is one which does not convert fully into hydroxide ions in solution (Clerk, J. 2013). pH is a logarithmic measure of hydrogen ions and is equal to a number which is either equal or between 0 and 14. If the number is higher then 7, the solution would be basic, however if the solutions pH is lower then 7, the solution is acidic, and if the solution is equal to 7, the solution is neutral (Rayner-Canham, et al., 2002). Physical properties of hydrochloric acid are it is a corrosive, colourless, transparent, aqueous solution, with a molar mass of 36.46 g/mol. The unknown base was a colourless, transparent, aqueous solution with a molar mass unknown (van Kessel., et al., 2003).

Procedure

        In this experiment, 25 mL of an unknown weak base was measured. Once measured the unknown weak base was measured it was poured into a 100 mL beaker. 2 drops of bromocresol green indicator was added. A burette with 1.00 mol/L HCl solution was set up. The pH of the unknown base solution was measured and recorded. Then the titrant was added in increments of 0.50 mL, while the pH was recorded between each additional 0.50 mL. The colour of the solution was also recorded at every addition of the titrant.

Results

Figure 1: Colours of pH during the Titration

[pic 1]

Figure 2: Volume of the HCl titrated into the weak base and its pH

Discussion        

        For this titration the estimated endpoint pH would be 3.94.1 This is the point on the figure 2 where the colour indicator changed from blue to yellow in colour. Using the information from the textbook, the equivalence point of the titration is in the middle of the steepest drop in pH (van Kessel., et al., 2003), which according to figure 2 would be after 25 ml of HCl were added making the pH equal to 2.17.2 The buffering region is the part of the graph where there is the least amount of change in pH, which occurs, using the data from figure 2, would be from 2.0 mL  to 24.0 mL.3 The balanced chemical equation for this neutralization reaction would be4;

B(aq) + HCl(aq)    HB+(aq) + Cl-(aq)

        The initial concentration of the weak base solution is 1.00 mol/L.5 the pOH of the weak base before any HCl was added is 5.13 and the concentration of [OH-] is 7.4x10-6.6 The chemical equation for the weak base at equilibrium would be displayed as7;

B(aq) + H2O(l)  HB+(aq) + OH-(aq)

The Kb for the unknown weak base is 5.5x10-118. Using the Kb calculated in question 8, the closest similar Kb value for the weak base would be the Kb for formate ion, which is predetermined to have a Kb of 5.6x10-11 (Acid-Base Ionization Constants).9 Considering the calculated value of 5.5x10-11, formate (HCO2), would be the best fit to identify the unknown weak base.

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